Non-destructive material evaluation refers to any of a wide variety of techniques that may be utilized to examine materials for defects and/or evaluate the materials without requiring that the materials first be destroyed. Such non-destructive material evaluation is advantageous in that all materials or products may be tested for defects. After being evaluated, acceptable (e.g., substantially defect-free or with acceptable defect levels) materials may be placed in service, while the defective materials may be re-worked or scrapped, as may be appropriate. Non-destructive evaluation techniques are also advantageous in that materials already in service may be evaluated or examined in-situ, thereby allowing for the early identification of materials or components that may be subject to in-service failure. The ability to evaluate or examine new or in-service materials has made non-destructive material evaluation techniques of great importance in safety- or failure-sensitive technologies, such as, for example, in aviation and space technologies, as well as in nuclear systems and in power generation systems.
One type of non-destructive evaluation technique, generally referred to as positron annihilation, is particularly promising in that it is theoretically capable of detecting fatigue and other types of damage in metals, composites, and polymers at its earliest stages. While several different positron annihilation techniques exist, as will be described below, all involve the detection of positron annihilation events in order to ascertain certain information about the material or object being tested.
By way of background, complete annihilation of a positron and an electron occurs when both particles collide and their combined mass is converted into energy in the form of two (and occasionally three) photons (e.g., gamma rays). If the positron and the electron are both at rest at the time of annihilation, the two gamma rays are emitted in exactly opposite directions (e.g., 180° apart) in order to satisfy the requirement that momentum be conserved. Each annihilation gamma ray has an energy of about 511 keV, the rest energies of an electron and a positron.
In positron annihilation analysis, the momentum of the electron is related to the environment in which it resides. For example, electron momentum is relatively low in defects in metals or in microcracks in composite materials and polymers) or in large lattice structures. Electron momentum is higher in defect-free or tight lattice structures. One way to determine the momentum of the electron is to measure the degree of broadening of the gamma-ray energy peak in the spectrum around the 511 keV annihilation energy produced by the annihilation event. Alternatively, the momentum of the electron may be derived from the deviation from 180° of the two annihilation gamma rays.
Additional information about the electron density of the material at the site of annihilation may be obtained by determining the average lifetime of the positrons following a known initiation event before they are annihilated. Still other information about the annihilation event may be detected and used to derive additional or supplemental information regarding the material being tested, such as the presence of contaminants or pores within the material. Accordingly, the detection of positrons and the products of annihilation events provide much information relating to defects and other microstructural characteristics of the material or object being tested.
As mentioned above, several different positron annihilation techniques are known. In one type of positron annihilation technique, positrons from a radioactive source (e.g., 22Na, 68Ge, or 58Co) are directed toward the material to be tested. Upon reaching the material, the positrons are rapidly slowed or “thermalized.” That is, the positrons rapidly loose most of their kinetic energy by collisions with ions and free electrons present at or near the surface of the material. After being thermalized, the positrons then annihilate with electrons in the material. During the diffusion process, the positrons are repelled by positively-charged nuclei, thus tend to migrate toward defects such as dislocations in the lattice sites where the distances to positively-charged nuclei are greater. In principle, positrons may be trapped at any type of lattice defect having an attractive electronic potential. Most such lattice defects are so-called “open-volume” defects and include, without limitation, vacancies, vacancy clusters, vacancy-impurity complexes, dislocations, grain boundaries, voids, and interfaces. In composite materials or polymers, such open-volume defects may be pores or microcracks.
Positron annihilation techniques utilizing external positron sources suffer from a variety of disadvantages. For example, one type of external positron source is an isotopic source, such as 22Na, which emits positrons having generally low energies of about 0.5 million electron volts (MeV) or so. Such low energy positrons can only penetrate a short distance, e.g., less than about 10 microns or so, into metallic materials. Such positrons also lose energy in the source material itself. While higher energy positrons (e.g., having energies of about 3 MeV) can be obtained via non-isotopic sources, such as the Pelletron at Lawrence-Livermore Laboratories, such devices are also not without their disadvantages. For example, the positron beams produced by such sources are often relative narrow, thus cannot easily be made to cover larger specimens. In addition, such external positron sources tend to be physically large, which can limit the ability to place the positron source at the appropriate location on the specimen to be tested. This is particularly true if the specimen comprises a fabricated structure (e.g., a wing structure) that comprises small areas or regions that are simply not large enough to accommodate the large external positron source. Consequently, it may be difficult, if not impossible, to effectively test such structures.